The invention relates to an injection valve for internal combustion engines, with a filter element arranged on the inside of the injection valve in the fuel flow of the latter and having ducts for the fuel.
Patent literature (WO 93/02284, EP 0472 417 A1, U.S. Pat. No. 5,050,569, U.S. Pat. No. 5,758,826 or U.S. Pat. No. 5,179,927) discloses a series of injection valves of this type which are also provided for the heating of fuel prior to injection into the combustion space of internal combustion engines. However, all the approaches used here for heating the fuel are distinguished by a comparatively large mass to be preheated. They therefore all have a high energy consumption and a long response time  greater than 60 s.
An object of the invention is to develop an injection valve and a method, by means of which, particularly when the engine is cold, improved mixture formation and as small a fraction as possible of CH in the exhaust gas during the starting phase can be achieved.
This object of the invention is achieved by providing an injection valve for internal combustion engines, with a filter element arranged on the inside of the injection valve in the fuel flow of the latter and having ducts for the fuel, wherein the filter element is designed as a throughflow heating element, and wherein the walls of the throughflow ducts are capable of being heated, at least in certain regions, along their longitudinal extent. The invention is distinguished by an extremely reduced thermal mass of the filter element which is embodied as a heating element. As a result, the response time for heating is reduced to a few seconds and the peak energy consumption is minimized (200 Wxe2x86x9220 W).
Thus, in particular, by virtue of the use of silicon as heater material and the shaping of the latter by means found in microsystem or semiconductor technology, it is possible to produce very large surfaces (inner surface  greater than 300 cm2 in the case of a heater area of 1 cm2) for the exchange of energy between the fuel and the heating element.
PTC resistors have hitherto been used as heating elements, and they maintain a fixed surface temperature of the heater without external regulation. On account of this, it has not been possible hitherto to regulate the heating capacity in an active way. This means that it is impossible to regulate the heating capacity, for example so as to adapt to varying fuel properties or quantities.
By contrast, the present invention makes it possible to have rapid response times in the seconds range, so that the heating element can be designed with out the disadvantage of additional waiting times.
Furthermore, the heating elements according to the invention are so small that they can be integrated into a conventional injection valve, specifically without the external dimensions of the injection valve having to be changed.
In order to measure the heating function of a filter element according to the invention, heptane, on the one hand, and water, on the other hand, were conveyed through the latter. The respective filter element had a diameter of approximately 10 mm. The diameter of a web between two adjacent ducts amounted to approximately 20 xcexcm, the duct length to approximately 300 xcexcm and the duct diameter to approximately 90 xcexcm.
In this filter element, it was possible to achieve a maximum throughflow of about 870 l/h at a water pressure of 6 bar. The achievable heating capacities were between 13 and 35 Watt.
With this set of parameters and a throughflow of about 2 l/h, it was possible for the liquid conveyed through to be heated by 30 to 50xc2x0 C. within 10 s to 20 s.
Surprisingly, despite its crystalline and therefore brittle material, the filter element did not exhibit any impairments in the case of fluctuations in the pressure of the liquid conveyed through. Consequently, also surprisingly, the mechanical stress on the material of a filter element according to the invention due to a slight pressure drop in a duct is uncritical and, in general, negligible.
Furthermore, local overheating possibly occurring within a duct can be ignored, since the heat in the filter element is distributed very quickly on account of the high thermal conductivity of the semiconductor material.
Expediently, an inventive, in particular semiconducting filter element can be used not only for heating, but also as a temperature sensor. For this purpose, preferably, the electrical resistance of the filter element developed as a heating element is determined (preferably when it is not heated) and is compared with an (in particular, predetermined) characteristic curve representing the temperature and/or resistance profile.
The relation between the fuel temperature and the electrical heating capacity makes it possible, by intelligent evaluation, to obtain further information, for example on the boiling point of the fuel and consequently, inter alia, the quality of the latter.
This possibility is based, inter alia, on the fuel forming bubbles at the boiling point. As a result of this bubble formation, the transmission of heat from the filter element into the fuel is lower. Consequently, the filter element is subject to greater specific heating, which can be detected not only from a certain temperature rise, but, for example, also from a significant change in the current/voltage characteristic curve and therefore also in the delivered heating capacity.
In this case, that is to say at the start of bubble formation due to the evaporation of the fuel, the measured heating capacity, preferably determined from the current/voltage graph, deviates from a theoretical value of the heating capacity which would occur in the case of a uniform transmission of heat into the fuel. To prevent evaporation, the heating capacity can then be reduced, for example, at least by an amount which causes the measured heating capacity to be again within a predeterminable range of the theoretical heating capacity.
In a simple way, in order to determine the boiling point, in particular the temperature of the filter element can be measured, at least indirectly, since the evaporation of the fuel is associated with rapid or sudden temperature rise of the filter element. A lowering of the temperature by several degrees was observed experimentally at the boiling point of the fuel.
By means of each of these simple measures, the fuel can be heated, regardless of its respective composition, up to or just before its boiling point, as a result of which, in particular, pollutant emission during a cold-starting phase is permanently improved.
Since the boiling point of the fuel used depends on its composition or quality, the pressure and the temperature, etc., this is particularly advantageous, since heating, of course, takes place solely to, at most, the actual boiling point of the fuel flowing through the filter element at a given time.
Furthermore, by means of the filter element according to the invention, in particular by a comparison with predetermined calibrating curves, for example, the following variables can also be determined:
I) the fuel flow in relation to the cooling of the filter element during the throughflow of fuel,
II) the fuel quality by determining the boiling point of the fuel, and
III) the pressure in the fuel system by means of a pressure displacement of the boiling point.
By means of the filter element according to the invention in the injection valve, in addition to the behaviour during cold starting being optimized, improvements can also be achieved in use during normal operation.
Thus, in particular, in a direct-injection petrol engine, a check of mixture formation in the combustion space, for example by controlling the depth of penetration of the fuel, that is to say controlled variation or keeping it constant in the case of different fuel compositions, is of substantial interest.
The heating or overheating of the pressurized fuel in the injection valve allows an explosive atomization of the fuel in the event of a pressure drop; that is to say during the opening of the needle of the injection valve.
Thus, the thermal control of fuel made possible by the invention has the same effects in terms of jet pattern and depth of penetration as other solutions of substantially more complicated design, such as, for example, electrostatic droplet-influencing devices or mechanically adjustable swirl plates in the vicinity of the valve orifice.
Further expedient refinements may be found in the subclaims. Moreover, the invention is explained in more detail with reference to exemplary embodiments illustrated in the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.